The decision comes as more states consider the issue, giving it a significance beyond California. Legislatures or courts in five other states – most recently in Iowa and Maine – have legalized same-sex marriage. New Hampshire and New York are weighing similar laws.

The California court will determine if Prop. 8, approved by 52 percent of voters last year, is an illegal revision of the state constitution. In addition, the justices are likely to determine the validity of some 18,000 same-sex marriages performed before last November's poll. The decision is expected to be posted on the court's website at 10 a.m. Pacific Time.

Gay marriage was legalized in California last year when the state's Supreme Court ruled, in a 4-to-3 decision, that a state law limiting marriage to a man and woman violated constitutional rights of gays and lesbians. While the ruling led to a wave of same-sex marriages in the state, it also gave rise to the $83 million campaign to amend the constitution that resulted in Prop. 8.

Though the state's justices ruled in favor of gay marriage last year, they may be reluctant to overrule a popular vote, says Jenny Pizer, senior counsel for Lambda Legal, a leading gay-rights advocacy organization and part of the suit challenging Prop. 8. The court has rejected various attempts to undo statewide propositions, such as reinstating the death penalty. Moreover, the legal argument being employed by gay and civil rights groups has only twice succeeded in overturning voter initiatives.

These groups maintain that Prop. 8 goes too far in altering the constitution. Prop. 8 "was created to make an exception" to the equal protection clause of the state constitution, "and that is a protection that can't be changed by using the initiative power," argues Ms. Pizer of Lambda Legal. Such an alteration requires a two-thirds majority from state lawmakers, she says.

But supporters of Prop. 8 said, in a brief filed to the court, that "the people have the final word on what the California Constitution says and there is no higher legal authority within California to which the judiciary can appeal."

Many legal experts agree that the court will be wary of overturning a referendum. Some speculate that the court will uphold the ban on gay marriage but recognize the marriages that took place prior to November's vote.

If gay rights groups lose the battle against Prop. 8 Tuesday, their options would be limited. They could pursue a ballot initiative of their own, but Pizer acknowledges that gay and lesbian groups will face a tough battle to sway a public that has already spoken out against same-sex marriage. "It is certainly not easy for a small minority to have to go the ballot box and beg the majority for equal rights," she says.

Alternatively, they could push for a legislative solution. State lawmakers, however, have already approved bills legalizing same-sex marriage only to see them vetoed by Gov. Arnold Schwarzenegger (R).

It's unlikely the case could move to the US Supreme Court because the fundamental argument is over the state constitution, not the federal one.

Their greatest hope could lie in public opinion, which has become more accepting of gay marriage, according to polls. A nationwide Washington Post/ABC News poll released last month found that 49 percent of respondents say gay marriage should be legal. When that question was put to Americans three years ago, only 36 percent agreed with legalizing same-sex marriages.

"If the Supreme Court upholds Proposition 8, there will be demonstrations all over the country, and that's a measure of the momentum growing around this issue," Pizer adds.

Similarly, Prop. 8 supporters will fight back if their ballot is invalidated. The groups that rallied behind the measure would form a constitutional convention to revise the state's charter, one supporter told the Associated Press.